It is a continuing goal to improve the efficiency of processes that are conducted using chemical process towers. To this end, many different approaches have been undertaken.
Gas/liquid contact is performed using cross-flow trays situated within the tower. Tray design typically includes a deck across which liquid flows, and a downcomer to convey liquid from one tray to that immediately below. The deck is perforated with apertures so that gas rises through said deck and then bubbles through the liquid. Gas ascends through the apertures and contacts the liquid moving across the tray through the “active” area thereof. It is in this area that liquid and gas mix and fractionation occurs during separation by distillation. The liquid is directed onto the tray by means of an inlet downcomer from the tray above. The liquid moves across the tray and exits through another downcomer. It is the active area of the tray which most directly effects gas liquid contact and thus mass transfer efficiency.
During normal operation, little if any liquid descends through the perforations. The rate of liquid flow in the downcomers and the geometry of the tray are designed so that a liquid seal is achieved at the bottom of the downcomers and so little if any gas rises in the downcomers from the tray below.
Optionally, the apertures perforating the deck may have bubble caps or valves that allow the flow of gas and provide better gas/liquid contact.
There may be one, two, or more downcomers from a tray. When there are more than one downcomers from a tray, the liquid flow is divided toward each of the downcomers and so the distance a liquid flows across the deck toward a downcomer is reduced.
Towers are designed so that there is disengagement of gas from the liquid in the top area of the downcomer, so that bubbles do not get conveyed as froth to the tray below. A weir is used to control the weir crest, so as to regulate the depth of froth above the top of the weir. The liquid head within a downcomer depends to a large extent on the pressure difference between successive decks and to a lesser extent also on factors including friction losses. These and other design features of towers are described by, for example, Philip C. Wankat in “Equilibrium Staged Separations” published by Elsevier (1988).
Separation processes that can be performed in chemical process towers include distillation and absorption. The optimum design of a tower having trays ensures maximum throughput (i.e. capacity) and mass transfer efficiency. At high throughput there is a tendency for liquid to be entrained at high gas velocity. This reduces the capacity as well as efficiency of the tower, caused by the liquid blowing to the tray above. Similarly, there can be entrainment of bubbles in down-flowing liquid when the downcomer design does not allow effective disengagement of gas. This can result in downcomer backup and reduce tray capacity.
The maximum throughput is reduced if the downcomer capacity is reached before high entrainment occurs on the tray. It is an objective of the present invention to effectively improve the downcomer capacity.
Bannon in U.S. Pat. No. 4,954,294 issued in 1990 described an apparatus for sealing vapor/liquid contacting trays on start-up of a downcomer-equipped vapor/liquid contactor. The downcomer is divided into two portions so that, at start-up, liquid immediately flows down one portion to an area of the tray below where a weir retains that liquid so as to form a seal. When the tray is in normal operation the liquid mainly flows down the other portion of the downcomer. A limitation of this apparatus is that the active mixing area of the tray is essentially only the area above the deck, and the area occupied by the first portion of the downcomer is not within that active mixing area.
Chuang in U.S. Pat. No. 5,213,719 issued in 1993 described a gas-liquid contacting device for increased gas-liquid flow rate capacity. The apparatus has, in addition to the conventional downstream downcomer, an upstream downcomer to handle up to 15% of the liquid flow. The upstream downcomer has a perforated plate sealing the lower end at a height above the froth of the tray below. Again, the area occupied by the upstream downcomer is not within the active mixing area of the tray.
The apparatus of the present invention has superior performance when compared to all prior art devices. We will now describe two embodiments of the present invention being apparatus which has higher capacity for handling high liquid flow rates when compared with the prior art.